hmm, I think I have a take on this, but I'm sure its not one that's.. well I don't know what value it might be to you, so I hope you can tolerate my strangeness....
My all time favorite sound design guy is Walter Murch:
http://en.wikipedia.org/wiki/Walter_Murch
He did the sound work on THX1138 (yes, that's where THX 5.1 comes from) American graphite, and apocalypse now. The stuff he did on THX has a lot to do with the sound design for star wars. I recommend renting those movies and checking out the extras.
I don't have much of a traditional theory background, or at least not a conventional theory background. But I am kind of hip to experimentalists whom... are arguably some of the pioneers to sound design... or at least they are in my mind. People like John Cage, Xenokis, umm.. Avant Guard folks. The idea of music concrete..
Playing around with some of these ideas.. I find myself using digital instruments to emulate sound effects, I find myself thinking about ambient space, using field recordings and collage as compositional elements.. searching through a kind of figure ground ambiguity between the experience of "sound" versus the experience of "music" and how I can get you to see music as sound and sound as music, or maybe I should substitute noise for sound in that statement?
Not that any of this really helps... but maybe exploring some of these kinds of ideas might be a good starting place.
Here's a little video with Cage talking about his work:
http://youtube.com/watch?v=2aYT1Pwp30M Some people say he's the most important composer since Beethoven. If you see Cage's place in our musical transition as coming, in some sense, out of traditional classical, then you can see how this sorta thing could come out of what you're doing..
Here's an amusing clip with cage:
http://youtube.com/watch?v=SSulycqZH-U
Here's a clip of Walter Murch talking about the sound design work he did for THX
http://youtube.com/watch?v=_py6jVyOqUY
I don't know where you are in knowledge or skills, but here we are getting into something like "traditional - esk music production, and it's applications for sound design." So I imagine that developing these kinds of skill sets and integrating them into your compositional process, is probably something to look at.
Finally, I agree with the folks recommending Reaktor. Max/MSP (
http://www.cycling74.com/products/maxmsp ) is a good choice to. These are great tools for taking one sound and really FUBAR-ing them into something else again. I think along with this, you might want to start exploring a little sound synthesis..
So where I come at it is.. I think of Cage's stuff as "transfiguration of the the common place" which is a kind of idea in aesthetics surrounding some of the work from that period.. wether we are talking about Cage, Duchamp, Warhol, or others.. its a way, among other things, of showing the common place in a new light.
In a way this speaks to nature / reality as it is: Cage talking about music that doesn't sound like a conversation.. his work often having to do with indeterminacy, meaning that the outcomes were not know.. his compositions being something like a strategy for a result. In my mind all this is sort of one box of toys and ideas.
In another box is something like "composing." Composing having to do with "organizing sounds." In here we might have traditional music theory, or we might just as well think of composition as playing with legos, where we put the lego's together in any way that fits our fancy.... this facilitating more avant guard ideas... but a full spectrum between this and more traditional ideas.
Then there's this other box called "music production" which is like.. skills of a mix engineer, perhaps a recording engineer, perhaps sound synthesis, though I don't know if the latter fits into this box.
So you go to composing, and you just take things out of these different toy chests and apply them together in whatever way strikes your fancy. I guess this is my idea of sound design.
Though sound design is often about creating an atmosphere, or certain kinds of sonic textures... perhaps ones that work a certain way in the context of a film.
There's a whole art form for making sound effects. How do you make the sound of a gun shot? The Sound of Fire? Throw that in there to..
Well that's, more or less, how I come at it anyway. Hope that helps somehow.